Histine has Acidic R group?? But r group has amine..

[SaintJude]

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How is histidine's R group weakly acidic.

I thought an amine like structure is basic??

 

[Buttafuoco]

I'm thinking it's because of aromaticity. Look what would happen if the NH picked up a proton- it would no longer have a lone pair and thus it would become antiaromatic. It really doesn't want to do that.

 

[MedPR]

Histidine is one of the three basic amino acids.

 

[chiddler]

Wiki says pKa 6.5.

I do remember it as a basic amino acid.

?

 

[MedPR]

The pKa doesn't tell you anything more than what it will be at physiologic pH, and at physiologic pH, histidine is basic because it can donate electrons.

 

[chiddler]

The pKa doesn't tell you anything more than what it will be at physiologic pH, and at physiologic pH, histidine is basic because it can accept electrons.

can you explain, please. there isn't much room for electron acceptance at physiological pH. only for proton donation as far as I can tell.

 

[Samchik]

I think MedPR meant that histidine is basic because it can donate its lone pair of electrons on nitrogen (= accept a proton).

 

[pfaction]

His can act as an acid and a base. "His goes both ways". It can lose a proton at higher pH's, and accept one at lower pH's.

 

[MedPR]

The pKa doesn't tell you anything more than what it will be at physiologic pH, and at physiologic pH, histidine is basic because it can accept electrons.

can you explain, please. there isn't much room for electron acceptance at physiological pH. only for proton donation as far as I can tell.

I think MedPR meant that histidine is basic because it can donate its lone pair of electrons on nitrogen (= accept a proton).

Yea, I meant donate electrons (or accept protons). I started to type accept protons, then I started thinking "donate electrons".

Sorry for the confusion.

 

[MedPR]

His can act as an acid and a base. "His goes both ways". It can lose a proton at higher pH's, and accept one at lower pH's.

This is true, but can't all the acid/base amino acids do this? At pH 1 lysine can donate protons, at pH 15 lysine can accept protons.

 

[SaintJude]

Yes, but Kaplan supplied a titration curve that showed a plateau of pKa 6--designating the proton in the R group

How should I have known that the pKa was 6 and not...8?

 

[MedPR]

Yes, but Kaplan supplied a titration curve that showed a plateau of pKa 6--designating the proton in the R group

How should I have known that the pKa was 6 and not...8?

You don't have to know any pKa values for amino acids, except maybe ~2 and ~9 for the C and N terminal.